1. Field of the Invention
The present invention generally relates to power supplies for electronic devices and, more particularly, high current, low voltage loads of complex electronic devices such as computers and telecommunications devices which operate at high frequencies.
2. Description of the Prior Art
Recent developments in many types of electronic devices have caused power supply requirements to become much more stringent to the point that current power supply designs are becoming inadequate or impractical. For example, switching power supplies are well known for DC-DC conversion and operate by switching current from a relatively high voltage source which is then filtered, often using a series inductance, to maintain an approximate desired voltage on a bulk capacitor, often referred to as a filter capacitor. Switching frequency and/or duty cycle to control output voltage is determined by feedback of variation in voltage in a known manner. However, such feedback involves an unavoidable delay in the response to transient changes in load and limits accuracy of voltage regulation.
Thus, power supplies of this and other known types rely on the large capacitance value of the bulk or filter capacitor to supply potentially large transient currents. Such filtering with capacitors may be required in several stages (e.g. following the bulk capacitor) of a power distribution network where the network may branch or present a relatively long connection (such capacitors being referred to as decoupling capacitors) as well as at the point of load (where such capacitors are often physically located on the back of a CPU and referred to as packaging capacitors) such as at an integrated circuit package or even at various locations on an integrated circuit chip to minimize effects of voltage ripple and variation due to load transients and propagation thereof through the power distribution network from one portion of a circuit or device to another. The slew rate of current change is also compromised by the total capacitance presented by these filter and decoupling capacitors. Such effects are particularly evident in regard to current and foreseeable complex digital circuits such as microprocessors or telecommunications devices which operate at extreme clock speeds and where the load can change radically from one clock pulse to the next and parasitic inductance and capacitance of power connections may cause large changes in impedance of the power supply connections.
Moreover, the trend toward increased complexity and reduced size of integrated circuit chips has resulted in designs which operate at relatively lower voltages (e.g. less than 1V) and higher currents (e.g. 150 A or higher—also aggravated by increased numbers of active devices such as transistors formed on each chip) supplied through connections presenting potentially significant parasitic impedances, particularly as the trend toward higher clock frequencies continues. Further, as a matter of thermal management, some integrated circuits, notably microprocessors, specify a variation of supply voltage with load which is referred to as adjustable voltage positioning (AVP). Therefore, voltage regulation tolerance has become smaller and more stringent as well as more complex while the potential magnitude of current transients and voltage drops in power connections increases. It is projected that the capacitance requirements for bulk capacitors will increase by a factor of 2.5 for bulk capacitors, a factor of 13 for decoupling capacitors outside a device connection (e.g. at an IC socket or board) and a factor of 6 for so-called packaging capacitors within or closely associated with an IC package or on-chip to derive sufficient voltage regulation performance using current voltage regulator and filter design to meet current and foreseeable voltage regulation requirements. Such large increases in required capacitance can be better appreciated from the fact that increased filter and decoupling capacitance compromises slew rate and bandwidth of the voltage regulator due to increasing numbers of output capacitors for energy storage, thus requiring even greater capacitance to provide a given voltage regulation tolerance which is becoming more stringent, as alluded to above. Further, while numerous capacitor technologies are known for different ranges of capacitance value, required increases in capacitance of individual capacitors is invariably accompanied by increases in physical size of capacitors and larger capacitance values must often be implemented in technologies which require greater volume per unit of capacitance.
Other approaches to answering a need for increased current slew rate and improved transient response are summarized in an article entitled “Hybrid Power filter with Output Impedance Control” by Y. Ren et al., 36th Conference on Power Electronics Specialists, pp. 1434-1440, Jun. 12, 2005, of which the inventors are co-authors and which is hereby fully incorporated by reference. However, each of these approaches has engendered corresponding problems and/or limitations rendering them inadequate to the requirements for providing power to current microprocessor and communications ICs. For example, a hybrid power supply noted therein provides a linear source in parallel with a switching regulator with the voltage loop closed through the linear source and the current from the linear source used as an error signal for the switching regulator. However, such an arrangement is inherently incompatible with adaptive voltage positioning (AVP) specified by manufacturers of microprocessors and other ICs of high functionality.